What is Lock-in Amplifier?

해당 글은 AMETEK사의 What is Lock-in Amplifier? 글을 번역한 글입니다.

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In its most basic form a lock-in amplifier is an instrument with dual capability. It can recover signals in the presence of an overwhelming noise background or, alternatively, it can provide high resolution measurements of relatively clean signals over several orders of magnitude and frequency. However, modern instruments offer far more than these two basic functions and this increased capability has led to their acceptance, in many scientific disciplines, as units which can provide the optimum solution to a large range of measurement problems. For example, the modern lock-in amplifier will function as:-

an AC Signal Recovery Instrument

a Vector Voltmeter

a Phase Meter

a Spectrum Analyzer

a Noise Measurement Unit

…..and much more

It is this versatility, available in a single compact unit, which makes the lock-in amplifier an invaluable addition to any laboratory. This Technical Note describes the basic “building blocks” of the lock-in amplifier so that the user and potential user may better understand how the instruments work and how the choices made in their design affect their performance

 

Introduction

A lock-in amplifier, in common with most AC indicating instruments, provides a DC output proportional to the AC signal under investigation. In modern units the DC output may be presented as a reading on a digital panel meter or as a digital value communicated over a computer interface, rather than a voltage at an output connector, but the principle remains the same. The special rectifier, called a phase-sensitive detector (PSD), which performs this AC to DC conversion forms the heart of the instrument. It is special in that it rectifies only the signal of interest while suppressing the effect of noise or interfering components which may accompany that signal. The traditional rectifier, which is found in a typical AC voltmeter, makes no distinction between signal and noise and produces errors due to rectified noise components. The noise at the input to a lock-in amplifier, however, is not rectified but appears at the output as an AC fluctuation. This means that the desired signal response, now a DC level, can be separated from the noise accompanying it in the output by means of a simple low-pass filter. Hence in a lock-in amplifier the final output is not affected by the presence of noise in the applied signal. In order to function correctly the detector must be “programmed” to recognize the signal of interest. This is achieved by supplying it with a reference voltage of the same frequency and with a fixed phase relationship to that of the signal. This is most commonly done by ensuring that they are derived from the same source. The use of such a reference signal ensures that the instrument will “track” any changes in the frequency of the signal of interest, since the reference circuit is “locked” to it. It is from this characteristic that the instrument derives its name. This inherent tracking ability allows extremely small bandwidths to be defined for the purpose of signal-to-noise ratio improvement since there is no frequency “drift”, as is the case with analog “tuned filter/rectifier” systems. Because of the automatic tracking, lock-in amplifiers can give effective “Q” values (a measure of filter selectivity) in excess of 100,000, whereas a normal bandpass filter becomes difficult to use with Q’s greater than 50.

 

Phase-Sensitive Detection

As mentioned above, the heart of the lock-in amplifier is the phase-sensitive detector (PSD), which is also known as a demodulator or mixer. The detector operates by multiplying two signals together, and the following analysis indicates how this gives the required outputs. Figure 1 shows the situation where the lock-in amplifier is detecting a noise-free sinusoid, identified in the diagram as “Signal In”. The instrument is also fed with a reference signal, from which it generates an internal sinusoidal reference which is also shown in the diagram.

The demodulator operates by multiplying these two signals together to yield the signal identified in the diagram as “Demodulator Output”. Since there is no relative phase-shift between the signal and reference phases, the demodulator output takes the form of a sinusoid at twice the reference frequency, but with a mean, or average, level which is positive.

Figure 2 shows the same situation, except that the signal phase is now delayed by 90° with respect to the reference. It can been seen that although the output still contains a signal at twice the reference frequency, the mean level is now zero.

From this it can be seen that the mean level is:-

proportional to the product of the signal and reference frequency amplitudes

related to the phase angle between the signal and reference

가장 기본적인 Lock-in Amplifier(이하 락인)의 유형은 두개의 기능성을 갖춘 장비이다.

1. 락인은 심한 노이즈가 있는 상태에서 신호를 잡아낼 수 있고,

2. alternatively, it can provide high resolution measurements of relatively clean signals over several orders of magnitude and frequency.

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현대의 장비들은 위의 상기한 기본적인 기능 외에 다양한 추가 기능을 제공한다.

1. an AC Signal Revocery Instrument

2. a Vector Voltmeter

3. a Phase Meter

4. a Spectrum Analyzer

5. a Noise Measurement Unit

6. .... and much more.

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한개의 락인 안에 위의 다양한 기능을 갖고 있는것이, 바로 락인을 모든 연구실에서 필수적으로 갖추어야할 장비인 이유이다.

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이 Technical Notes에서는 락인의 간단한 bulding blocks를 설명한다. 이를 통해 유저들이 해당 장비가 어떻게 작동하고, 기능설정이 퍼포먼스에 어떤 영향을 끼칠지에 잘 이해하게 될 것이다.

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Introduction

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